The present invention relates to lift cranes, and more particularly to connectors for coupling adjacent segments or sections of a column, such as a column used as a boom for cranes and the like.
Large capacity lift cranes typically have elongate load supporting column structures, commonly used for boom, mast, or jib, that comprise sectional column members secured in end-to-end abutting relationship. Predominantly, each of the column members is made of a plurality of chords and lacing or lattice elements. The terminal end portions of each chord are generally provided with connectors of one form or another to secure abutting column segments together and to carry compressive loads between abutting chords. Typical connectors comprise one or more extensions and plates secured by a pin carrying compressive loads in double shear.
An example 220 foot boom may be made of a 40 foot boom butt pivotally mounted to the crane upper works, a 30 foot boom top equipped with sheaves and rigging for lifting and supporting loads, with five sectional boom members in between: one 10 feet in length, one 20 feet in length and three 40 feet in length. Such an example boom has six boom segment connections. Typically each segment has four chords, and hence four connectors, making a total of 24 connectors that must be aligned and pinned to assemble the boom.
Typically, the loads carried by the boom members and, consequently, through the connectors require the lugs, also referred to as extensions, on the connector to be sufficiently thick to have sufficient strength to bear the loads. To carry very high loads for a high capacity crane, a typical single extension sandwiched between two plates, giving a double shear connection, requires a very large pin diameter to carry the compressive loads and, consequently, requiring the connectors to be very large. Standard specification plate steel often is insufficiently thick to form the extensions on a connector having sufficient strength to support the loads. For example, 100,000 pound per square inch (100 kpsi) plate steel is available in 4 inch thick plates and 130 kpsi plate steel is available in 2¾ inch thick plates, but neither is sufficiently thick in itself to form a connector capable of carrying the highest loads. While higher strength steel plates of greater thickness may be available, obtaining it typically requires a special order with a steel mill at commensurately higher costs and lead times. As a consequence, the connectors typically are formed of cast steel so as to have a sufficient thickness and strength.
Casting a connector, however, poses several challenges and inefficiencies. First, qualifying a foundry, preparing a mold, and casting a connector are a time intensive and, consequently, costly processes. Indeed, a long lead time and significant work may be invested in preparing a mold before the first connector can be cast. Provided a production run is sufficiently large it may make sense to mold many connectors, but only a small number of the largest cranes with the largest connectors in terms of both size and overall number may be manufactured.
Further, because of the long lead times and high costs of casting, the process is not easily adaptable to engineering and design changes, prototype testing, and the manufacture of one or a small number of components for use in destructive testing or as replacement parts. Stated differently, as a manufacturing process, the process of casting connectors often is not sufficiently agile and adaptable to rapidly changing business conditions and requirements.
Another disadvantage of cast connectors is that casting defects are not uncommon. As a consequence, a cast connector may require finish work or machining to ensure that a connector falls within the required specification and tolerances for a given application. This finish work often can be time consuming and expensive, too.
As a result, there exists a need for a connector that is quicker and easier to manufacture than a cast connector.